State achieved retroactively by superstars before anybody knew who they were. If you're watching a 10-year-old movie on late-night television and see someone in a supporting or fringe role who didn't "burst on the scene" till five years ago, the person you're watching in the movie is "pre-famous."
Pre-fame can only come after the fact. It is different from mere obscurity. Whatever their publicists say, not everyone you haven't heard of is pre-famous.
Watching pre-famous people often gives insight into their genuine personalities and talents that their later celebrity conceals. It's much easier to imagine them excited about the prospect of success and hanging out with real people after the performance. Also, the pre-famous often have to do a lot more nudity than the famous.
There are multiple routes to celebrity, and there are likewise several different kinds of pre-fame.
In music, singers and musicians often serve in back-up roles for established stars before striking out on their own. Sheryl Crow was once a back-up singer for Michael Jackson, and if you listen carefully, you can hear her voice on some earlier recordings.
Just about every movie actor starts out playing Friend of Perky Girl or Frat-boy #4. These trivial characters are usually meant to remain in the background, but as soon as you spot a pre-famous person in such a role, the illusion is destroyed. In fact, it's often impossible to pay attention to anything other than the pre-famous actor's head-bobbings and mindless exclamations, whatever the primary scripted exchange is supposed to be.
Television pre-fame is about the same, except that pre-famous television actors guest-star on established series, rather than having minor roles in one-shot productions.
If you look for roles in which a major star was pre-famous and can't find any, it's usually safe to believe that star won't be famous for long. The pre-famous are talented but unproven; the famous who lack pre-fame are usually celebrated not because of talent but because they happen to possess some quality that is currently but transiently popular.